Method of preparing negative electrode active material

This application claims priority from Korean Patent Application No. 10-2019-0175725, filed on Dec. 26, 2019, the disclosure of which is incorporated by reference herein.

The present invention relates to a method of preparing a negative electrode active material.

Recently, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, demand for secondary batteries with relatively high capacity as well as small size and lightweight has been rapidly increased. Particularly, since a lithium secondary battery is lightweight and has high energy density, the lithium secondary battery is in the spotlight as a driving power source for portable devices. Accordingly, research and development efforts for improving the performance of the lithium secondary battery have been actively conducted.

In general, the lithium secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, an electrolyte, and an organic solvent. Also, with respect to the positive electrode and the negative electrode, an active material layer including a positive electrode active material or a negative electrode active material may be formed on a current collector. A lithium-containing metal oxide, such as LiCoOand LiMnO, is generally used as the positive electrode active material in the positive electrode, and, accordingly, a carbon-based active material or silicon-based active material containing no lithium is used as the negative electrode active material in the negative electrode.

A passivation film, such as a solid electrolyte interface layer (SEI layer) formed of LiF or LiCO, is formed on a surface of the negative electrode active material during initial charge of the negative electrode. Since the passivation film prevents the organic solvent from being inserted into the negative electrode and inhibits a decomposition reaction of the organic solvent, it improves stabilization of a negative electrode structure and reversibility of the negative electrode, and allows it to be used as a negative electrode.

However, since LiCO, as a component of the solid electrolyte interface layer, has very low ion conductivity, there is a problem in that diffusion of lithium ions is concentrated on LiF or an interface of LiF and LiCOand this causes overcharge and a resistance increase phenomenon in the corresponding part, and thus, there is a problem in that a lithium precipitation phenomenon occurs. The precipitated lithium may significantly reduce stability of the negative electrode active material, for example, penetration of the passivation film, a rapid exothermic reaction, and degradation of life performance, while growing in the form of dendrites.

Accordingly, there is an urgent need to develop a negative electrode active material in which stability is improved while ionic conductivity of a surface of the negative electrode active material is made uniform.

Korean Patent Application Laid-open Publication No. 10-2017-0074030 discloses a negative electrode active material for a lithium secondary battery and a method of preparing the same, but has limitations in solving the above-described problems.

[Patent Document]

Korea Patent Application Laid-open Publication No. 10-2017-0074030

An aspect of the present invention provides a method of preparing a negative electrode active material with improved rapid charging performance and life performance.

According to an aspect of the present invention, there is provided a method of preparing a negative electrode active material which includes the steps of: (a) dispersing an active material core in a solution containing a surfactant to coat the surfactant on the active material core; (b) adding and dispersing a first precursor, which is bondable with the surfactant by electrostatic attraction, in the solution; (c) adding and dispersing a second precursor, which is bondable with the first precursor by electrostatic attraction, in the solution; (d) preparing a lithium compound precursor by a hydrothermal reaction of the first precursor and the second precursor in the solution; and (e) performing a heat treatment on the lithium compound precursor to thermally decompose the surfactant, and forming a protective layer containing a lithium compound on the active material core, wherein one of the first precursor and the second precursor is at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium nitrate, and lithium sulfate.

According to a method of preparing a negative electrode active material of the present invention, a process of coating an active material core with a surfactant is performed before a lithium compound-containing protective layer is formed on the active material core. Since the surfactant may form a uniform charge on a surface of the active material core, it may be bonded to an ionic or polar lithium compound protective layer precursor by electrostatic attraction. Since the bonding by the electrostatic attraction allows the lithium compound to be uniformly coated on the surface of the active material core, ionic conductivity and life performance of the negative electrode active material may be significantly improved.

Also, according to the method of preparing a negative electrode active material of the present invention, since a specific lithium compound, instead of a solid electrolyte interface layer (SEI layer) containing LiCOor the like, is formed on the active material core, a phenomenon, in which inflow/outflow of lithium ions are concentrated in a specific region of the active material core, and a phenomenon, in which lithium in the form of dendrites is precipitated, may be prevented, and thus, stability and life performance of the negative electrode may be improved.

Furthermore, according to the method of preparing a negative electrode active material of the present invention, rapid charging performance of the negative electrode may be improved by forming a protective layer containing a lithium compound having high ionic conductivity on the active material core.

It will be understood that words or terms used in the specification and claims shall not be interpreted as the meaning defined in commonly used dictionaries, and it will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. In the specification, the terms of a singular form may include plural forms unless referred to the contrary.

It will be further understood that the terms “include”, “comprise”, or “have” when used in this specification, specify the presence of stated features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

The expression “average particle diameter (D)” in the present specification may be defined as a particle diameter at a cumulative volume of 50% in a particle size distribution curve. The average particle diameter (D), for example, may be measured by using a laser diffraction method. The laser diffraction method may generally measure a particle diameter ranging from a submicron level to a few mm and may obtain highly repeatable and high-resolution results.

Hereinafter, the present invention will be described in detail.

<Method of Preparing Negative Electrode Active Material>

The present invention provides a method of preparing a negative electrode active material, specifically a method of preparing a negative electrode active material for a lithium secondary battery.

Specifically, the method of preparing a negative electrode active material according to the present invention includes the steps of: (a) dispersing an active material core in a solution containing a surfactant to coat the surfactant on the active material core; (b) adding and dispersing a first precursor, which is bondable with the surfactant by electrostatic attraction, in the solution; (c) adding and dispersing a second precursor, which is bondable with the first precursor by electrostatic attraction, in the solution; (d) preparing a lithium compound precursor by a hydrothermal reaction of the first precursor and the second precursor in the solution; and (e) performing a heat treatment on the lithium compound precursor to thermally decompose the surfactant, and forming a protective layer containing a lithium compound on the active material core, wherein one of the first precursor and the second precursor is at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium nitrate, and lithium sulfate.

LiF or LiCOis contained in a solid electrolyte interface layer (SEI layer) that is formed during initial charge of the negative electrode active material, wherein since LiCOamong them has very low ion conductivity, there is a problem in that inflow/outflow of lithium ions are concentrated on LiF or an interface of LiF and LiCO. Current concentration in a specific region of the negative electrode active material causes a problem such as overcharge and resistance increase in the corresponding part, and causes a problem in which lithium is precipitated in the form of dendrites. The precipitation of the lithium may significantly reduce stability of a negative electrode, for example, penetration of the solid electrolyte interface layer, explosion of the negative electrode, and degradation of life performance.

According to the method of preparing a negative electrode active material of the present invention, a surfactant is dispersed on a surface of an active material core so that the surface of the active material core has a positive (+) charge or a negative (−) charge, and a first precursor, which is an ionic material or a polar material having a charge opposite thereto, is added and dispersed so that the first precursor and the surfactant are bonded by electrostatic attraction and the first precursor is uniformly disposed on the active material core. Since the first precursor may be uniformly disposed on the active material core, a protective layer containing a lithium compound may be uniformly formed on the active material core by adding a second precursor, a hydrothermal reaction step, and a heat treatment step which will be described later. Since the protective layer may improve ionic conductivity and rapid charging performance of the negative electrode active material and may prevent a material having low ionic conductivity, such as LiCO, from being formed on the active material core, the lithium precipitation and the resulting problems of degradation of stability and life performance of the negative electrode may be addressed.

Also, the negative electrode active material prepared by the method of preparing a negative electrode active material of the present invention is characterized in that a protective layer containing a specific lithium compound having high ionic conductivity is formed on the active material core. Since the protective layer containing a specific lithium compound is formed in advance instead of the solid electrolyte interface layer (SEI layer) formed during the initial charge of the negative electrode active material, the ionic conductivity of the negative electrode active material is improved and a phenomenon, in which lithium is concentrated in a specific region, may be prevented. Furthermore, since the precipitation of the lithium may be prevented by forming the protective layer on the active material core, the stability of the negative electrode, particularly, the life performance of the negative electrode may be significantly improved.

The method of preparing a negative electrode active material according to the present invention includes the step of (a) dispersing an active material core in a solution containing a surfactant to coat the surfactant on the active material core.

The active material core may be at least one selected from the group consisting of a carbon-based material and a silicon-based material. Specifically, the active material core may be a carbon-based material in terms of exhibiting excellent cycle characteristics and battery life performance.

The silicon-based material may include a compound represented by SiO(0≤x<2). Since SiOdoes not react with lithium ions, it may not store lithium, and thus, x may be within the above range and more preferably, the silicon-based material may be SiO.

The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon, and may preferably include at least one selected from the group consisting of artificial graphite and natural graphite.

An average particle diameter (D) of the active material core may be in a range of 1 μm to 30 μm, for example, 5 μm to 20 μm in terms of ensuring structural stability during charge and discharge and reducing a side reaction with an electrolyte solution.

The surfactant may be uniformly coated on the active material core by dispersing the active material core in a solution containing the surfactant, and the surface of the active material core is allowed to have a positive (+) charge or a negative charge (−) so that it may contribute to the uniform and smooth formation of the protective layer.

The surfactant may be a cationic surfactant or an anionic surfactant.

The cationic surfactant may include at least one selected from the group consisting of cetyltrimethylammonium bromide (CTAB), triethylemine hydrochloride, benzothonium chloride, and cetylpyridinium chloride, and may preferably include cetyltrimethylammonium bromide in terms of excellent solubility in water and being more environmentally friendly by containing Bras an anion.

The anionic surfactant may include at least one selected from the group consisting of ammonium lauryl sulfate, sodium lauryl sulfate, sodium laureth sulfate, α-olefin sulfonate, and sodium stearate, and may preferably include sodium lauryl sulfate in terms of excellent solubility in water and low price.

The surfactant may be coated on the active material core such that a portion having a positive (+) charge or a negative (−) charge faces the outside of the active material core. Specifically, a portion having hydrophobicity in the surfactant is in contact with the surface of the active material core, and the portion having a positive (+) charge or a negative (−) charge (for example, ammonium ion of the CTAB) may be disposed to face the outside of the active material core.

The dispersion of the active material core may be performed by adding the active material core to the solution and performing an ultrasonic treatment.

The method of preparing a negative electrode active material according to the present invention includes the step of (b) adding and dispersing a first precursor, which is bondable with the surfactant by electrostatic attraction, in the solution, and the step of (c) adding and dispersing a second precursor, which is bondable with the first precursor by electrostatic attraction, in the solution. In this case, one of the first precursor and the second precursor includes at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium nitrate, and lithium sulfate.

The first precursor and the second precursor may be a material having ionicity or strong polarity, wherein, since the first precursor may be bonded to the above-described surfactant by electrostatic attraction and the second precursor may be bonded to the first precursor by electrostatic attraction, the surface of the active material core may be evenly coated. Since one of the first precursor and the second precursor is a material containing lithium, the first precursor and the second precursor may form a lithium compound-containing protective layer uniformly coated on the surface of the active material core through the hydrothermal reaction step and the heat treatment step, and, accordingly, lithium precipitation and the resulting problems of degradation of the stability and life performance of the negative electrode may be addressed by not only preventing the lithium precipitation due to local intercalation/deintercalation of lithium, but also preventing the formation of the material having low ionic conductivity, such as LiCO, on the active material core.

In a case in which the surfactant is the cationic surfactant, the first precursor may be at least one selected from the group consisting of hydrogen sulfide (HS), nitric acid (HNO), hydrofluoric acid (HF), silicic acid (HSiO), sulfuric acid (HSO), and phosphoric acid (HPO), and the second precursor may be at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium nitrate, and lithium sulfate.

In the case that the surfactant is the cationic surfactant, a compound having an anion is first added as the first precursor to the solution so that the cationic surfactant and the first precursor may be bonded by electrostatic attraction. The first precursor may be uniformly disposed on the surface of the active material core by the electrostatic attraction with the cationic surfactant. Since the first precursor reacts with the second precursor, selectively the second precursor and a third precursor to form a lithium compound, the protective layer containing a lithium compound may be uniformly formed on the active material core.

Specifically, in the case that the surfactant is the cationic surfactant, the first precursor may provide at least one anion selected from sulfide ions (S), nitrate ions (NO), fluoride ions (F), silicate ions (SiO), sulfate ions (SO), and phosphate ions (PO) into the solution, and the anion may form a bond with the cationic surfactant by electrostatic attraction.

In the case that the surfactant is the cationic surfactant, the first precursor may be at least one selected from the group consisting of hydrogen sulfide (HS), nitric acid (HNO), hydrofluoric acid (HF), silicic acid (HSiO), sulfuric acid (HSO), and phosphoric acid (HPO), and may be preferably phosphoric acid (HPO).

In the case that the surfactant is the cationic surfactant, the second precursor may be at least one selected from the group consisting of lithium hydroxide (LiOH or LiOH.HO), lithium oxide (LiO), lithium nitrate (LiNO), and lithium sulfate (LiSO), and may be preferably lithium hydroxide. The second precursor may provide lithium ions to the solution, and the lithium ions may be bonded to the anions derived from the first precursor by electrostatic attraction. Since the ions derived from the first precursor and the second precursor may be uniformly coated on the active material core by the electrostatic attraction, the lithium compound-containing protective layer may be uniformly coated on the active material core by the hydrothermal reaction and heat treatment to be described later, and thus, rapid charging performance and life characteristics may be improved at the same time.

The second precursor may be at least one selected from the group consisting of LiOH.HO, LiO, LiNO, and LiSO, and may be preferably LiOH.HO.

In the case that the surfactant is the cationic surfactant, in step (C), a third precursor, which includes at least one metal selected from the group consisting of lanthanum (La), zirconium (Zr), titanium (Ti), aluminum (Al), and germanium (Ge), more preferably at least one metal selected from titanium and aluminum, and most preferably titanium and aluminum, may be added and dispersed.

The metal in the third precursor, for example, may exist in the form of a metal ion or in a form having a partial positive (+) charge in the solution, and the metal ion may be distributed around the anion derived from the above-described first precursor by electrostatic attraction. Accordingly, the metal in the third precursor may be included in the lithium compound to become a protective layer component by the hydrothermal reaction step and heat treatment step to be described later.

The third precursor may be at least one selected from the group consisting of a nitric oxide, an alkoxide, a sulfide, and an oxide of the metal.

In a case in which the surfactant is the anionic surfactant, the first precursor may be at least one selected from the group consisting of lithium hydroxide, lithium oxide, lithium nitrate, and lithium sulfate, and the second precursor may be a compound including at least one selected from the group consisting of hydrogen sulfide (HS), nitric acid (HNO), hydrofluoric acid (HF), silicic acid (HSiO), sulfuric acid (HSO), and phosphoric acid (HPO).

In the case that the surfactant is the anionic surfactant, a compound capable of providing lithium ions is first added as the first precursor to the solution so that the anionic surfactant and the lithium ions derived from the first precursor may be bonded by electrostatic attraction. The first precursor may be uniformly disposed on the surface of the active material core by the electrostatic attraction with the anionic surfactant. Since the first precursor reacts with the second precursor, selectively the second precursor and the third precursor to form a lithium compound, the protective layer containing a lithium compound may be uniformly formed on the active material core. Since the ions derived from the first precursor and the second precursor may be uniformly disposed on the active material core by the electrostatic attraction and the lithium compound-containing protective layer may be uniformly coated on the active material core by the hydrothermal reaction and heat treatment to be described later, the rapid charging performance and life characteristics may be improved at the same time by preventing a problem of the lithium precipitation due to a localized lithium displacement phenomenon and preventing the formation of an SEI layer component, such as LiCOhaving low ionic conductivity, on the active material core.